As the next step we introduce light-cone coordinates and momenta [1]}, [2]}, [3]}
\(& &x^-=\phi -t \ , \quad x^+=\frac{1}{2}(\phi +t)+\alpha x^- \ ,\nonumber \\& &p_+=p_\phi +p_t \ , \quad p_-=\frac{1}{2}(p_\phi -p_t)-\alpha p_+ \ , \nonumber \\\) 
